(a) Field of the Invention
The present invention relates to a communication node system, a control node system, and a communication system using those node systems in an Ethernet-PON (Passive Optical Network).
(b) Description of the Related Art
Ethernet-PON (hereinafter referred to as “EPON”) is a passive optical subscriber network designed for applying the Ethernet used in the existing LANs (Local Area Networks) to general subscriber networks. EPON is comprised of the PON that has passive splitters for combining, in a passive manner, an OLT (Optical Line Termination) connected to the network with ONUs (Optical Network Units) connected to subscribers. Namely, the PON is a medium shared tree structure, that unlike the conventional media sharing in the Ethernet, has a directionality of data transmission due to the characteristics of fiber and splitters such that a down-stream frame sent from the OLT is forwarded to all the ONUs, while an up-stream frame is sent directly to the OLT from one ONU without being forwarded to another ONU.
The EPON uses an approach for communicating Ethernet frames between an OLT connected in the PON form and a plurality of ONUs, and it is now standardized as IEEE 802.3ah. The existing Ethernet supports shared medium and point-to-point links. A PON in which down-stream data are broadcast and up-stream data are forwarded only to the OLT based on TDM (Time Division Multiplexing) is a network not supported in the existing MAC (Media Access Control).
With the upper bridge connected in the PON, a problem occurs in regard to compatibility. Namely, when an ONU sends a MAC frame to an OLT and the MAC frame is forwarded to another ONU, the upper bridge must send the frame to the same port again, and accordingly, the ONU again receives the MAC frame at the same port as used for sending the MAC frame. This is unacceptable in the existing bridge. For the purpose of compatibility with the existing bridge, the PON structure has an LLID (Logical Link Identification) for logical identification of the ONU, inserted into the 8-byte preamble to acquire compatibility with the upper bridge. Using the LLID, the EPON system appears as multiple point-to-point links in a point-to-multipoint communication environment so as to be compatible with the upper bridge. The LLID in the down-stream frame represents the ONU to which the frame is forwarded (LLID may include “broadcast”), and the LLID in the up-stream frame represents the ONU from which the frame is forwarded.
In the existing Ethernet, all the terminals or nodes belonging to the same LAN segment receive the same frames at the same time. In the PON, for forwarding a frame from one ONU to another, the ONU sends the frame to the OLT and then the OLT returns the frame to the corresponding ONU with its destination address. But the frame received at the OLT cannot be returned to the ONU according to the existing Ethernet standard. To solve this problem in the EPON, some approaches have been attempted, as follows.
In the first approach, an emulation layer for point-to-point communication is provided between a physical (PHY) layer and a MAC layer. This approach is subdivided into two methods. In one method, the emulation layer removes a PON-tag from all the frames received from the ONU, sends the removed PON-tag to the upper layer, and broadcasts a copy of the frame to all the ONUs through a PON interface. Each ONU checks the PON-tag of the received frame. The ONU discards the frame by filtering if the PON-tag is identical to its PON-tag; otherwise, it removes the PON-tag and sends it to the upper layer. However, this method that returns all the frames to the PON interface even in the case of non-point-to-point communication between ONUs has a problem in regard to an excessive consumption of the down-stream bandwidth in the PON.
In the other method, the PON-tag attached to the up-stream frame from each ONU is used to discriminate the frame, which is then forwarded to the upper MAC layer. As many logic MAC layers are provided as there are ONUs connected to the PON interface. Each MAC layer is connected to the port of the bridge so that the frame is switched through the bridge and forwarded down through the corresponding MAC layer. The frame forwarded from the bridge is combined with the PON-tag corresponding to the destination address and is broadcast to the ONU through the PON interface. The ONU checks the PON-tag of the received frame, and removes the PON-tag and sends the frame to the upper layer if the PON-tag is identical to its PON-tag; otherwise, it discards the frame by filtering. In this case, each MAC layer performs frame multiplexing, and inter-MAC layer multiplexing must be performed once again, thus making the frame multiplexing complicated. Additionally, this method must provide as many logical MAC layers as there are ONUs for one PON interface.
In the second approach, a shared LAN emulation layer is provided on the MAC layer of the OLT. The shared LAN emulation layer is comprised of three parts, i.e., a lower layer shared LAN emulation layer, a logical MAC layer, and an upper layer shared LAN emulation layer. The lower layer shared LAN emulation layer checks the LLID of the up-stream frame from the ONU and sends the frame to the MAC layer. The logical MAC layer sends the frame to the upper layer shared LAN emulation layer. The upper layer shared LAN emulation layer, which has a similar function to the bridge of the IEEE 802.1D, checks the LLID of the frame for point-to-point communication and sends the frame to the corresponding logical MAC layer. The logical MAC layer sets the LLID to the frame and sends the frame to the lower layer. For broadcasting, the frame is forwarded down with a separate broadcasting logical MAC, in which case broadcasting bits are set. Here, the number of the logical MACs is k (the number of LLIDs×2+1), and the frame format must be different from the standard for the purpose of the shared LAN emulation function. The precise frame format is not mentioned in the cited reference of this technology. Moreover, the need for multiplexing between logical MACs increases the complexity of the multiplexing for the down-stream data.